% state(me,ce,mw,cw)

%safe(Me,Ce,MW,CW) :- Me >= Ce, MW >= CW.

unsafe(Me,Ce,MW,CW) :- Me > 0, Ce > Me.
unsafe(Me,Ce,MW,CW) :- MW > 0, CW > MW.

% move 2 missionaries from east to west
move(state(Me,Ce, Mw, Cw), state(ME,CE,MW,CW)) :-
        Me > 1, ME is Me-2, MW is Mw+2, CW = Cw, CE = Ce,
        not(unsafe(ME,CE,MW,CW)).

% move 2 cannibals from east to west
move(state(Me,Ce, Mw, Cw), state(ME,CE,MW,CW)) :-
        Ce > 1, ME is Me, MW is Mw, CW is Cw+2, CE is Ce-2,
        not(unsafe(ME,CE,MW,CW)).